The present invention relates generally to mixture control systems for an internal combustion engine, and in particular to a closed-loop mixture control system using diagnosis circuits for testing the functions of the control loop.
In a closed-loop mixture control system, an exhaust composition sensor is provided to supply information on the air-fuel ratio of the mixture and feeds its information to a control circuit having proportional and integral control response characteristics to generate a signal that varies the fuel quantity such that the air-fuel ratio is controlled at a desired value.